The present invention relates to an instrument for measuring the static viscoelasticity and/or dynamic viscoelasticity of a material.
A known invention of this kind is an instrument for measuring the relation between the stress and strain in a material as described in Patent Publication No. 57-40963 (hereinafter referred to as the first prior art). Also, as described in Patent Laid-Open No. 63-139232 (hereinafter referred to as the second prior art instrument), an improved instrument is available. The mechanisms of this instrument are simplified to increase the mechanical strength and to decrease the weight of the vibrating system. Thus, the effects of the mechanical resonance are mitigated. Hence, the dynamic viscoelasticity of the material can be measured. In addition, as described in Patent Laid-Open No. 3-82934 (hereinafter referred to as the third prior art instrument), another improved instrument applies an alternating force to a material. A DC current is added to the alternating force. The DC strain component of the strain developed in the material is mechanically compensated. The dynamic viscoelasticity of the material can be measured only from the AC component by the pulling system.
In all of these prior art instruments, a force is applied to a sample via a detection rod. The strain in the sample is detected by a displacement detector placed between the detection rod and an external support. The detection rod of these instruments needs to be supported by a support within the instrument of some form. In the first prior art instrument, a balance support mechanism is used. In the second and third prior art instruments, leaf springs are mounted to the supports.
In each prior art instrument described above, the friction between the detection rod and the support must be small (i.e., the viscous resistance between the detection rod and the support is small). This is a characteristic generally required for each system for holding the detection rod.
In addition, in order to measure the static viscoelasticity of the sample, the elastic coupling constant (spring constant) between the detection rod and the support) is required to be small.
To measure the dynamic viscoelasticity of a sample, the mass of the vibrating portion including the detection rod must be small, forminimizing the measurement error due to inertia. Furthermore, the resonant frequency of the vibrating portion determined by the ratio of the elastic coupling constant to the mass of the vibrating portion needs to be higher than the measured frequency and so it is necessary that the elastic coupling constant between the detection rod supports be considerably large.
In particular, measurement of the static viscoelasticity of a sample and measurement of the dynamic viscoelasticity are common in that the relation between a stress and a strain produced in the sample is measured. However, where the detection rod is held as in the prior art technique described above, conflicting requirements take place concerning the elastic coupling constant between the detection rod and the support. Consequently, any instrument capable of accurately measuring both static and dynamic viscoelasticities of a sample has not existed.
In practice, the static viscoelasticity can be measured, using the first prior art instrument. However, measurement of the dynamic viscoelasticity is limited to quite low frequencies of less than 1 Hz due to the large mass of the balance mechanism.
On the other hand, in the second and third prior art instruments, it is possible to measure dynamic viscoelasticity up to higher frequencies such as hundreds of Hz. In measuring the static viscoelasticity of a sample, it is difficult to separate the contribution of the spring constant of leaf springs when the stress and strain vary at the same time.
In consequence, sufficient measuring accuracy cannot be obtained.